1. Technical Field of the Invention
The present invention relates to a fusing unit which fuses an nonfused toner image formed on a sheet medium and includes a heating roller, a pressing roller pressed to the heating roller, a heat-resistant belt which is wrapped around an outer periphery of the pressing roller and travels while being nipped between the heating roller and the pressing roller, and a belt stretching member for stretching the heat-resistant belt and an image forming apparatus.
2. Description of the Related Art
The following two types of fusing devices have been proposed as a heating-roller-type fusing device which is mounted on an image forming apparatus such as a copier, a printer, or a facsimile and which fuses a nonfused toner image on a transfer material through contact thermofusing. Namely, one type of fusing device comprises a heating roller whose surface is coated with an elastic body and which has a built-in heat source and is capable of rotating; an endless heat-resistant belt stretched by a plurality of support rollers; and a pressing member which wraps the heat-resistant belt around the heating roller over only a predetermined angle to thus form a nipping portion and which locally applies pressure, which is greater than that applied to the other areas, to the heat-resistant belt at an exit of the nipping portion, to thus cause distortion in the elastic body on the surface of the heating roller. The fusing device facilitates output of the sheet medium from the nipping portion (see, e.g., Japanese Patent No. 3084692). The other type of fusing device has a pressing member which has a projecting section and is provided at the inside of the endless heat-resistant belt, thereby decreasing a minute pressure area in the nipping portion (see, e.g., Japanese Patent No. 3480250).
The fusing devices require a plurality of support rollers and rotary bearings thereof. This renders the fusing device expensive as well as complicated and bulky; and inevitably makes an image forming apparatus equipped with the fusing device complicated, bulky, and expensive. Further, when the circumference of the heat-resistant belt becomes longer and the belt is moved over a predetermined path, the heat-resistant belt is deprived of thermal energy by a plurality of support rollers, and the amount of naturally-dissipated heat is increased in accordance with the circumference. Accordingly, a longer time must be consumed before the temperature of the heat-resistant belt reaches a predetermined level. This undesirably entails a longer so-called warm-up time which elapses from when power is turned on until fusing becomes feasible.
The heat-resistant belt is wrapped around the heating roller over only an angle which enables formation of a nipping portion, and pressure which is greater than that applied to the other areas is locally applied to the heat-resistant belt at the exit of the nipping portion, to thus cause distortion in the elastic layer of the heating roller. This configuration is suitable for preventing the sheet medium from wrapping around the heating roller. However, the sheet medium output along the distortion of the elastic layer is curled in imitation of this distortion or is subjected to deformation, such as occurrence of wrinkles, caused by local high pressure.
In addition to these fusing devices, another fusing device (see, e.g., Japanese Patent Publication No. 6-40235B) has also been proposed. The device deforms rollers by the pressure set between the rollers, to thus form a nipping length over which a sheet medium is to contact the rollers. A sheet medium carrying a nonfused toner image is caused to pass through the nipping portion, thereby fusing the toner image. The rollers are driven by selecting a first speed or a second speed as a drive speed of the rollers in accordance with characteristics of the sheet medium. However, the heat capacity of the rollers is large, and, hence, consumption of a long warm-up time is undesirably required. In addition, the sheet medium, having passed through the long nipping portion formed by deforming the rollers with pressure, undergoes stress derived from the pressure, as in the case of the former fusing device, which in turn causes deformation of the sheet medium, such as occurrence of a curl or wrinkles.
Still another fusing device (see, e.g., Japanese Patent Publication No. 2004-4235A) has been proposed as a device which solves the above-described drawbacks. A stretching member is placed at a position which is upstream with respect to the moving direction of a heat-resistant belt and where the heat-resistant belt turns itself around a heating roller to thus form a nipping portion, with reference to a tangential line of a press contact position defined between the heating roller and a pressing roller. This stretching member is supported so as to be swayable. As a result, the structure of the heat-roller-type fusing device can be subjected to simplification, downsizing, and cost-saving. A warm-up time can be shortened, and deformation of an output sheet medium, such as occurrence of a curl or wrinkles in the sheet medium, can be prevented by reducing stress imposed on the sheet medium.
The structures of the respective related-art fusing devices that have been proposed thus far are effective means for enhancing the fusing characteristic. However, the structures are not sufficient for forming a stable nipping portion in the axial direction of the heating roller and that of the pressing roller; namely, over the entire longitudinal area of the heating roller and that of the pressing roller. More specifically, the nipping portion is slightly relevant to axial deflection of the heating roller and that of the pressing roller attributable to pressure or deformation of the pressing member or that of the stretching member, such as a twist, a warpage, or an axial torsion. For these reasons, under the present circumstances, the nipping portion is under influence of such deformation, and difficulty is encountered in preventing the nipping portion from becoming unstable. Thus, enhancement of the fusing characteristic cannot be achieved.
In addition, there may arise a case where creeping deformation arises under influence of the heating roller which is heated to a temperature as high as about 200° C. Particularly, when the stretcher is formed from a plastic material having low thermal capacity, on the assumption that the stretcher is effective for shortening a warm-up time, the phenomenon of occurrence of deflection or deformation has become noticeable, which is not preferable for forming a stable nipping portion.